Losses in Armored Single-Conductor Lead-Covered A-C. Cables

The losses occurring in single-conductor armored cables whose sheath and armor are bonded and grounded at more than one point are¿besides the copper loss and the dielectric loss¿the circulating-current losses in the sheath and in the armor, and the additional iron losses in an armor of magnetic material. The circulating-current losses are due to the currents induced in the sheath and armor circuits by the fluxes linking these circuits when these circuits are closed by the bonds or grounding connections. Since the losses obtained in such cables are widely variable, depending on the cable design, both designers and operating engineers desire to know the loss magnitudes obtained in different cases and to have an understanding of the factors tending to give low losses. In this paper the sheath and armor losses have been analyzed for a steel-wire armored cable, a copper-wire armored cable, a steel-tape armored cable, and a cable enclosed in an iron pipe, and have been compared with the losses occurring in a plain lead-covered cable without armor. The data used include test data and calculated values. It is shown that a steel-wire armored 350,000-circular mil single-conductor copper cable with sheath and armor short circuited by low-impedance bonds had a total loss (exclusive of dielectric losses), of 2.8 times the conductor loss, by test in a single-phase 60-cycle circuit at 4 ft. cable spacing, at 260 amperes. The corresponding loss in a similar cable without armor, with lead sheath short circuited, was 2.4 times the conductor loss.